Marine risers are widely used in the process of hydrocarbon extraction from subsea oil wells. The marine riser extends from a BOP stack located on the seabed up to an oil vessel located on the surface. During intervention operations a length of tubing called a workover riser is located within the marine riser. The workover riser passes through the BOP stack via a well control device, normally consisting of dual well isolation valve and a disconnect system, and subsequently via the production tubing down to the formation. Completion and intervention activities within the well bore are performed from the surface vessel via the workover riser.
Conventionally, a compensator system is provided on the vessel. The compensator system has two main functions. The first is to apply a force to the workover riser to maintain the workover riser in a substantially constant tension.
As the prevailing sea conditions can cause the vessel to oscillate vertically with respect to the seabed, the compensator system's second function is to compensate for the vertical oscillation to ensure the tension in the workover riser remains substantially constant. If the compensator system does not compensate adequately for the movement of the vessel due to, for example, a system lock-up, then as the vessel moves vertically away from the seabed, the workover riser can become over-tensioned possibly inducing tensile failure. Similarly, as the vessel moves towards the seabed, the workover riser will enter a compressive state possibility inducing compressive failure.
Under flat sea conditions, the upper end of the workover riser is located at a sufficient height above the vessel deck to ensure that, in rough seas when the vessel is at the maximum extent of its vertical movement, the upper end of the workover riser is still maintained above the level of the vessel deck.
As conventional compensation systems support the workover riser from above they have to, therefore, be mounted in a derrick high above the surface of the deck. If it is necessary to perform an operation downhole requiring, for example, the introduction of tools into the workover riser, an operator may have to be lifted up to the upper end of the riser in an operation called man-riding. When an operator is man-riding he is effectively suspended above the deck and is exposed to potential falling hazards or impact injuries, particularly in strong winds. In this situation, the operator is also exposed to the additional danger of becoming snagged or trapped in the surface well control equipment as it oscillates in relationship to the vessel.
If the workover riser suffers tensile failure, both considerable HSE risks and severe environmental damage can occur when the contents of the workover riser are released.
It is also known to provide weak link systems which permit separation of the vessel from the workover riser in the event of failure to minimise damage to equipment. Conventional weak link systems do not, however, prevent the contents of the workover riser being released.
Furthermore, when the workover riser fails it is desirable to isolate and seal the well. In the event of workover riser failure, the BOP rams generally have to cut through the workover riser, a situation which is not ideal as it is preferable for the rams to be unobstructed as they close.